The present invention relates to an AC generator for vehicles loaded on cars, trucks and so on.
The engine room size has been narrower recently due to the slanted nosings for reducing vehicle running resistance and the need for securing a residential space of compartments, and there is no room for a loading space for an AC generator for vehicles. On the other hand, the number of revolutions of the engine is lowered and the rotation of the AC generator for vehicles is also lowered for improving fuel cost, whereas an increase in electric loads such as safety control appliances is requested, resulting in demands on increasing enhancement of generation performance. Further, power has been increased while the engine is miniaturized and reduced in weight, and as a result, vibrations increase, and vibrations applied to a generator loaded on the engine also increase.
Further, engine noises have been lowered recently due to the social need for reduction in noise outside the vehicle and the improvement in quality of the goods in terms of enhancement of silence of compartments. In particular, magnetic noise of the AC generator for vehicles caused by generation is offensive to the ear.
As described above, it is desirable to provide at low cost an AC generator for small vehicles that, has a high output, is excellent in resistance to vibration, and is low in noise production.
On the other hand, in the conventional AC generator for vehicles, when a stator is held and secured to both front and rear frames, a plurality of flanges are provided in a direction of external diameter at a position in which both the frames are opposed in the vicinity of an end of an opening of the frame in contact with a stator core 32, the flanges being formed with bolt hole or threaded holes and fastened and secured by means of through-bolts. In the above-described construction, however, since the flanges of the frames are axially fastened at an external diametrical position of the frames, a moment component force M is applied as shown in FIG. 9 so that the end of the opening of the bowl-like frame becomes distorted, and an axial contact surface between the stator core and the frame reduces. Thereby, the surface pressure of the contact surface rises and therefore is shaved and worn by the edge of the end of the stator core which is formed of iron replaced by aluminum which has been generally used as material for frames. Under the current environment in which vibrations from the outside are applied to the generator as mentioned above, such wear is further accelerated. As the wear progresses, the axial distance between the frames shortens, and the through-bolts by which the stator core and the frames are fastened and secured axially become loosened, leading to the following problem. That is, the center axis between the rotor and the stator is deviated so that an air gap between the outer periphery of the rotor and the inner periphery of the stator becomes uneven, and a magnetic attraction thereof generated through the air gap also becomes uneven, whereby the magnetic noises increase.
Further, since the rigidity of the entire generator lowers sometimes the external vibrations and resonance phenomenon by which the stator core relatively rotates in a peripheral direction between the frames to break a connecting wire between a stator winding and a rectifier, leading to a danger of a stoppage of generation. Further, the vibrations of the entire generator is rapidly increased due to the resonance, whereby an excessive stress is applied to various parts such as a rectifier to possibly break them.
On the other hand, from the viewpoint of providing an inexpensive generator, the stator core of the generator is generally a helical type core formed by winding a band-like steel plate helically. In this case, both axial ends which are a beginning of winding and a termination of winding have the same inclination of 90xc2x0 with respect to the center axis. The contact surface between one axial end and the frame further reduces in contact surface, and in the other contact surface, it slides along the inclination of the contact surface of the stator so that the opening of the frame tends to spread, thus reducing the axial distance between both the frames. In the above-described helical type stator core, occurrence of looseness of the through-bolts is accelerated.
Further, as compared with the core in which separated steel plates are laminated, the helical type stator core is formed of a thin steel sheet for facilitating helical winding, and therefore the rigidity is lowered. Thereby, the stator core tends to be deformed by the fastening force of the through-bolts. There is also a problem of an increase in magnetic noise caused by deterioration of circularity of an inner periphery.
As a countermeasure against the above-described problem, there is employed a hot insert system in which an interference fit is used for fitting between the frame and the stator core, and when assembled, the frame is heated in advance, not merely depending on fixed through-bolts between the frame and the stator core.
It is also contemplated that the axial fastening force of the through-bolts is increased to thereby increase the securing force between the stator core and the frame. In this case, however, the moment component force M increases, and the end of the opening of the bowl-like frame is further distorted. It is therefore contemplated that a cylindrical outer peripheral portion of the frame is increased in thickness to increase the rigidity of the frame in an attempt to reduce the distortion.
On the other hand, by the increase of the moment component force M, the stator core in the vicinity of the through-bolts is deformed toward the inner peripheral side so that the circularity of the inner periphery of the core is deteriorated. This gives rise to a new problem in that an air gap between the outer periphery of the rotor and the inner periphery of the stator is uneven, and the magnetic noises increase. In the helical type stator core whose rigidity is low, this tendency is notable. To solve this problem, notches are provided in the vicinity of flanges of frames for fastening through-bolts to prevent deformation of the stator core, as disclosed in Japanese Patent Application No. Hei 7-245901.
In the above-described hot insert system, the number of production steps and equipment therefor increase, impeding a supply of an inexpensive generator. Further, when a temperature of the generator caused by generation rises, fitting becomes loosened due to a difference in expansion coefficient between aluminum which is a material for the frame and iron which is a material for the rotor core 32, and if sufficient interference fit is intended to be secured under the aforesaid conditions, a large tensile stress is conversely always applied to the frame when the vehicle stops to pose a problem of a crack of the frame in the cold district. If the frame is made of the same iron as the stator core 32, the looseness of fitting caused by the difference in expansion coefficient disappears but there poses problems of a lowering of output caused by leakage of magnetic flux, and a considerable increase in processing cost and weight. In the case where the helical type stator core is used, the rigidity lowers as described above. Therefore, there is a problem in that deformation due to the interference fit between the core and the frame increases, and the circularity of the inner periphery is deteriorated and as a result, the magnetic noises increase.
Further, in the method of making a wall-thickness larger to increase the rigidity of the frame in order to reduce the deformation and distortion of the opening of the frame. It is necessary for obtaining sufficient effects to considerably increase the thickness of outside diameter of the entire frame to pose a problem of increasing the loading space for the engine and the weight.
Furthermore, in the method of the above-described Japanese Patent Application No. Hei 7-245901, in the contact surfaces other than the flanges, the distortion of the end of the opening of the frame caused by the moment component force M and the occurrence of the wear of the contact surface accompanied thereby remain the same as prior art, and the problem of lowering the fastening force of the bolts is not solved. Further, the problem of the increase in magnetic noises caused by the deterioration of the circularity of the inner periphery in the case where the helical type stator core whose rigidity is low is used still exists.
The present invention solves the conventional problems as noted above. It is an object of the present invention to provide at low cost an AC generator for vehicles which prevents an occurrence of looseness of through-bolts, is small in size, has a high output, is excellent in resistance to vibration, and is low in noise.
For solving the above-described problems, in the generator according to the present invention, a recess is provided in an axial direction of an outer peripheral portion of a helical stator core, and an axial center position of a through-bolt guided in the recess is controlled to thereby considerably reduce a moment component when the through-bolt is axially fastened, to suppress a wear of a contact surface between a frame and the stator core, and to suppress an occurrence of looseness of the through-bolt as well as deformation of the helical stator core to prevent an increase in magnetic noise.
According to the first aspect of the invention, there is provided an AC generator for vehicles comprising, a field rotor having a fan at least on one side of an axial end, a stator arranged oppositely of the outer periphery of the rotor and comprising a laminated core having a plurality of slots on the inside diameter side and a yoke on the outside diameter side and a stator coil housed in the slot, a pair of bowl-shaped frames for supporting the rotor and the stator, and a through-bolt for axially fastening and fixing the stator by the pair of frames, characterized in that the stator coil is formed by helically winding and laminating a band-like steel plate to have an approximately cylindrical shape, the yoke has axial both ends placed in contact with an approximately end of an opening of a bowl-like portion of the frames, the approximately cylindrical stator core is formed in its outer peripheral surface with a plurality of axially depressed recesses, and an axial center of the through-bolt is arranged on or within a first prescribed circle, wherein said first prescribed circle has a diameter which is larger than a largest outside diameter of the stator core by 2xc3x97⅓ of the diameter of the through-bolt. A moment component caused by an axial fastening force of the through-bolt is considerably reduced, and a distortion of the end of the opening of the frame is suppressed. Therefore, an axial contact area between the stator core and the frame can be secured so that progress of a wear of the frame contact surface is prevented, the axial fastening force of the through-bolt is retained, and almost all of the axial fastening force acts for the purpose of fixing the stator core and the frames, thus enhancing the rigidity of the entire generator, and withstanding the recent severe environment of external vibration conditions from the engine. Particularly, in the case where the helically wound and laminated helical type stator coil is used, further reduction of one frame contact area caused by inclination of both ends in the axial direction of the core and easiness of spreading caused by sliding of the other frame opening are overcome. The retaining effect of the axial fastening force of the through-bolt is further enhanced as compared with the conventional fastening and fixing construction. Further, since deformation caused by the lower rigidity of the helical type stator core is suppressed, magnetic noises caused by deterioration of circularity of the inner periphery are not increased.
According to the second aspect of the invention, there is provided the AC generator for vehicles according to the first aspect of the invention, wherein the recess is approximately semicircular, and the through-bolt is circular in section, and wherein the through-bolt is guided by the semicircular recess.
Since the outside diameter of the through-bolt is guided along the recess when the through-bolt is mounted, the external thread is easily inserted into the internal thread. It is not necessary to apply a large guide to the opening of the internal thread, resulting in facilitating the mounting and processing.
According to the third aspect of the invention, there is provided the AC generator for vehicles according to the first or second aspect of the invention, wherein the laminated core is joined by welding along the axial direction in the recess surface. The rigidity of the recess on which a compressive force acts due to the fastening of the through-bolt is further enhanced to prevent deformation of the stator core 32 whereby an increase in magnetic noise caused by the defective circularity of the inner periphery disappears, and a build-up of weld is not present in the vicinity of the contact surface between the core and the frame, thus enhancing the mounting workability.
According to the fourth aspect of the invention, there is provided the AC generator for vehicles according to any one of the first to third aspects of the invention, wherein the outer peripheral surface of the through-bolt is extremely close to the recess in a portion opposed to the recess. A magnetic line of force is caused to pass through the through-bolt whereby a magnetic path in the stator core 32 is enlarged as compared with the case where the outer peripheral surface of the through-bolt is not close to the recess, thus enhancing the output.
According to the fifth aspect of the invention, there is provided the AC generator for vehicles according to any one of the first to fourth aspects of the invention, wherein the stator coil is that at least a portion positioned in the slot is approximately rectangular along the slot shape, and the slot is approximately filled with a plurality of coils. Since the rigidity of the entire stator is enhanced, deformation of the stator core caused by the fastening of the through-bolt and defective circularity of the inner periphery of the stator core resulting therefrom are prevented not to increase magnetic noises.
According to the sixth aspect of the invention, there is provided the AC generator for vehicles according to the first aspect of the invention, whereby an axial center of the through-bolt is arranged on or out of a second prescribed circle, wherein said second prescribed circle has a diameter which is smaller than the largest outside diameter of the stator core by 2xc3x97⅔ of a diameter of the through bolt. The moment component caused by the axial fastening force of the through-bolt is considerably reduced whereby progress of a wear of the frame contact surface is prevented, the axial fastening force of the through-bolt is held, the deformation of the helical type stator is suppressed to eliminate an increase in magnetic noises, and in addition, lowering of output caused by reduction in magnetic path area in a magnetic circuit can be suppressed.